This invention relates to output power control devices for automotive AC generators.
FIG. 13 is a circuit diagram showing a conventional output power control device for an automotive AC generator. A output power control device similar to that of FIG. 13 is disclosed, for example, in Japanese Utility Model Publication (Kokoku) No. 62-30480. In FIG. 13, the AC output of the AC generator 1 including the armature coil 101 and the field coil 102 is rectified by a full-wave rectifier 2 including a main output terminal 201, an auxiliary output terminal 202, and a grounded terminal 203. The output of the AC generator 1 is regulated to a predetermined voltage level by a voltage regulator circuit 3 including: voltage divider resistors 301 and 302 coupled in series, a Zener diode 303 coupled to the junction point between the resistors 301 and 302, a control transistor 304, a power transistor 305 for turning on and off the field current through the field coil 102, a resistor 306 and a surge absorber diode 307. The circuit of FIG. 13 further includes: a battery 4 charged by the AC generator 1, a key switch 5, and a resistor 6 for supplying the initial excitation current to the field coil 102. The current supply to the electric load 7 of the vehicle is controlled by a load switch 8.
The operation of the circuit of FIG. 13 is well known. Namely, when the key switch 5 is made before starting the engine, the base current for the power transistor 305 is supplied from the battery 4 through the key switch 5 and the resistors 6 and 306, and the power transistor 305 is turned on. As a result, the field current flows from the battery 4 through the key switch 5, the resistor 6, the field coil 102 and the power transistor 305 to the ground, and a magnetomotive force is generated by the field coil 102.
When the engine is started and the AC generator 1 is driven, an AC output is induced across the armature coil 101 corresponding to the rpm thereof and is rectified by the full-wave rectifier 2. When the output voltage of the full-wave rectifier 2 is below a predetermined level (e.g., 14.4 V), the voltage at the junction point between the resistors 301 and 302 is below the break down voltage of the Zener diode 303. The control transistor 304 is thus kept turned off. The power transistor 305 continues to be turned on and supply of the field current to the field coil 102 is maintained. The output voltage of the AC generator 1 thus rises as the rpm of the engine increases.
When the output voltage of the AC generator 1 rises above the predetermined level (14.4 V) as a result of the increase in the rpm thereof, the voltage between the resistors 301 and 302 rises to turn on the Zener diode 303 and supply the base current to the control transistor 304. The control transistor 304 is thus turned on, thereby grounding the base of the power transistor 305. As a result, the power transistor 305 is turned off, so as to interrupt the field current supplied to the field coil 102. The output voltage of the AC generator 1 thus falls. When the output voltage falls below the predetermined level, the Zener diode 303 and the control transistor 304 are again turned off, and the power transistor 305 is turned on. The field coil 102 is thus energized and the output of the AC generator 1 again rises.
Repeating the above operation, the output of the AC generator 1 is controlled to the predetermined level (14.4 V). The battery 4 is thus charged to the predetermined voltage level.
As shown by the dotted curve C1 in FIG. 2E, the maximum output current that can be generated by the AC generator 1 increases as the rpm thereof increases, but is saturated at about 5000 rpm. Usually, the crossing point of the maximum output current (curve C1) with the total electric load of the vehicle (two-dots and chain curve L) is set about 2500 rpm.
The output power control device for an automotive AC generator thus has the following disadvantage. When the rotational speed of the AC generator 1 is less than 2500 rpm, the total load L of the vehicle is greater than the maximum output current C1 available from the AC generator 1. Thus, if, for example, the vehicle is trapped in a traffic jam and is forced to run continually for a prolonged time at a low speed at which the AC generator 1 is driven at less than 2500 rpm, the voltage level of the battery 4 tend to fall below the normal. In particular, during the night time when the headlights of the vehicle are turned on, or when the electric load of the vehicle is great, the battery may be over-discharged and the engine may be halted as a result.